This invention relates generally to brake systems in vehicles and more particularly to brake systems having both auxiliary drive line brakes and wheel brakes.
The term "auxiliary drive line" or "auxiliary brake" refers to brakes which are employed in the power train drive line from the engine to the driven wheels to assist the wheel brakes, as is known to be required in heavy vehicles such as buses and trucks. This function, which shall be referred to hereinafter as "auxiliary braking" requires a brake which may even have smaller force transfer capabilities than wheel brakes, but a much greater ability to absorb heat energy created during braking than wheel brakes.
Trucks and motor-coaches are being equipped with ever more powerful driving engines so that the mean operating speeds increase while it becomes impractical to enlarge the brakes in the wheels. In order to relieve the wheel brakes, so-called retarders have been employed in which the braking energy is converted electrically or hydrodynamically. Apart from the disadvantage of the comparatively great weight and large expenditure, such retarders have the drawback that their no brake moment is dependent on the driving speed and no linear characteristic corresponding to that of a friction brake operating without self-intensification. Self-servo is the tendancy of brake elements operating in a liquid medium to come together, because of hysteresis effects therein; as a result thereof, brake application does not vary linearly with the applied braking force. Their effect is virtually dependent on the square of the speed. The braking action of the retarder is superimposed upon the braking action of the wheel brakes during braking. The function of locking regulators, the provision of which will become preferable within the next few years, is then illusory on the wheels on which the retarder operates for the driver cannot be expected at the moment of emergency braking first to cut out the retarder before actuating the wheel brakes. A further disadvantage is the comparatively long release time of a retarder which does not meet the prerequisite most important for anti-locking regulation, viz, the capability of reducing the brake moment promptly.
Other proposals relate to brake systems which involve in addition to the wheel brakes a brake connected to the transmission system.
In these brake systems, the transmission brake is provided for emergency braking or parking applications (U.S. Pat. No. 2,140,040, British Pat. No. 1,143,793). However, this transmission brake is not suitable for auxiliary braking in the same manner as the drive line brake of the present invention. Another proposal is known (British Pat. No. 992,703), in which a brake system involves two different brakes wherein one is actuated before the other by a common pedal. The first so-called "retarding" brake was proposed to provide a coolant circulated disc brake having the rotating parts within the coolant. Such brake systems have never been realised in practice because of the difficulty in providing suitable auxiliary braking. Oil or coolant circulated disc brakes are not practical because of the comparatively low dissipation of the heat occurring during auxiliary braking by oil or other usual coolants. High heat dissipation requires large brake assemblies requiring a great expenditure apart from the necessity of providing coolant filters. Another disadvantage arises in the fact that the friction coefficient is dependent on the velocity. This causes a non-linear characterisitc and excludes the provision of anti-locking devices in a brake system of this kind so that no progress is achieved with respect to the use of the above retarder.
With the state of the art as exemplified by the foregoing, the main object of this invention is to provide a brake system which has one or more drive line brakes constituting an auxiliary brake means, and conventional wheel brakes. the auxiliary brake relieves the load on the wheel brakes and allows the provision of anti-locking devices in the brake system.
It is a particular object of this invention, to provide an auxiliary drive line brake which is designed for auxiliary actuation having at least twice the maximum brake energy as the wheel brakes. The auxiliary brake is a watercooled disc brake having dry friction parts, operating without self-intensification effect and having a linear brake characteristic. Friction self-intensification means that the braking effect of a brake is not linear with regard to the friction coefficient of the friction lining but depends on latter, raised to a higher power. A brake with high friction self-intensification is sensitive, as at the smallest fluctuation of the friction coefficient which can occur through entering of water and dirt and/or at high temperature, the braking effect is changed to such an extent that the whole distribution of braking power on each wheel is questionable. Therefore, brakes with high friction self-intensification are no longer used in modern vehicles. A brake without friction self-intensification whereby the braking moment is directly proportional to the friction value of the friction lining, does not show these disadvantages as the braking moment only changes proportionally with the friction value of the lining.
It is a further object of this invention to provide actuating means for the auxiliary drive line brake and the wheel brakes, including a singular application mean and separate circuit means. The actuating means operates said auxiliary drive line brake alone in a first mode of operation during which said wheel brakes remain unbraked and operates in a subsequent second mode of operation said wheel brakes in addition to said service brakes such that the total applied braking moment is distributed to each wheel approximately in proportion to the dynamic load distribution on that wheel.
A further object of the invention is the provision of adjusting means by which the auxiliary drive line brake is operated with a constant or slower increasing brake moment beginning with a predetermined retardation.
Another object of the invention is the provision of an anti-locking arrangement in combination with the brake system. The anti-locking arrangement includes means to control the auxiliary drive line brake and the wheel brakes simultaneously.
It is a further object of this invention to provide a brake system for a truck-trailer combination. Both the truck and the trailer are equipped with shaft brakes constituting the auxiliary drive line brakes. The auxiliary brakes are watercooled disc brakes operating without self-intensification and designed for auxiliary braking actuation. The combination is provided with a singular application mean by which actuation first the trailer auxiliary brake, than the truck trailer auxiliary brake and finally the wheel brakes are subsequently effected. This provides a stretched condition of the combination preventing jack-knife. The design of the auxiliary brakes allows the provision of anti-locking devices for all wheels.